This invention relates generally to a novel carbonaceous pitch. More specifically, the present invention is concerned with a reformed mesophase pitch useful as precursor materials for carbon fibers having a high strength and a high modulus.
As precursor materials for carbon fibers, polyacrylonitrile fibers have been hitherto used. Due to the expensiveness and poor carbon yield of the polyacrylonitrile fibers, however, the use of carbonaceous pitches which are inexpensive and provide a high carbon yield have been proposed as a substitute for the acrylonitrile fibers in recent years.
As precursor materials for carbon fibers, both optically isotropic and anisotropic pitches have been employed. Natural and synthetic pitches are generally isotropic in nature and afford isotropic carbon fibers with low-strength and low-modulus. On the other hand, anisotropic pitches can form carbon fibers having a strength and a modulus as high as those obtained from rayon or acrylic fibers. Therefore, the recent trend in the production of carbon fibers is towards the use on anisotropic pitches as starting materials.
Anisotropic pitches may be produced by thermal treatment of natural or synthetic pitches which are generally composed of condensed ring aromatics of average molecular weight of a few hundred or less, and which are isotropic in nature. When such isotropic pitches are heated to a temperature of about 350.degree.-450.degree. C., anisotropic, small spheres begin to appear in the matrix of the isotropic pitch as a result of cyclization, aromatization, polycondensation and like reactions of the aromatics. These small spheres, which are considered to be liquid crystals of a nematic structure, are composed of relatively high molecular weight hydrocarbons having a polycyclic, condensed ring structure and a high aromaticity and which are insoluble in quinoline. With an increase in heat treatment time or temperature, these small spheres gradually grow in size and coalesce with each other. As coalescence continues, the pitches become anisotropic as a whole, with a simultaneous increase in viscosity, and are finally converted into coke. The optically anisotropic, small spheres or their coalesced domains are termed "the mesophase" and pitches containing such material are termed "mesophase pitches". Conventional carbon fibers or anisotropic structures can be produced by spinning a mesophase pitch, rendering the spun fibers infusible and carbonizing the infusible fibers, as disclosed in Japanese Published Examined Patent Publication No. 49-8634, Japanese Published Unexamined Patent Application Nos. 49-19127, 53-65425, 53-119326 and 54-160427.
However, the production of carbon fibers from mesophase pitches has been found to involve certain difficulties. The fundamental problem arises in the spinning step and is mainly ascribed to the fact that the mesophase components of the pitch have higher melting points and, in the molten state, a higher viscosity than the components forming the isotropic matrix of the pitch. More specifically, when for spinning the mesophase pitch is heated to a temperature so as to melt the isotropic matrix but not to melt the mesophase components, the pitch becomes thixotropic because of the presence of the solid-like phase mesophase components and, therefore, smooth spinning is seriously inhibited. If, on the other hand, the spinning temperature is raised to a temperature permitting the melting of the mesophase components, then the mesophase components, which are thermally unstable, gradually increase in viscosity because of polymerization and tend to form coke. Especially in the case of mesophase pitch having a high mesophase content, such coking proceeds very fast, to the extent that a continuous spinning operation is considerably inhibited. Thus, although anisotropic carbon fibers derived from mesophase pitches have superior mechanical properties in comparison with isotropic carbon fibers obtained from isotropic pitches, the production of the anisotropic fibers inherently involves a problem in the spinning step.
To solve this problem, there has been proposed the use of a mesophase pitch having a relatively small molecular weight and a low quinoline insoluble content for the production of carbon fibers. The strength of the carbon fibers derived from such a mesophase pitch, however, is not satisfactory though the modulus thereof is improved as compared with those obtained from synthetic polymeric materials such as polyacrylonitrile fibers.